The subject matter disclosed herein relates to aircraft. More particularly, the present disclosure relates to management of fuel flow from and between aircraft fuel tanks.
Fuel costs are a large operating expense for aircraft operators such as airlines, so operators are constantly making efforts to reduce such costs. One way to potentially reduce an aircraft's fuel consumption is to manage the aircraft's longitudinal axial (i.e., “pitch” axis) center of gravity (CG) during long range flights.
It is known that during cruise flight conditions positioning the aircraft CG slightly aft along the aircraft's longitudinal axis can reduce the aircraft's drag, thus reducing fuel consumption. With the longitudinal CG positioned slightly aft, negative effects of tail plane lift are reduced, leading to a reduced overall angle of attack (OAA) of the aircraft due to a reduced moment arm impacted by diminished tail plane lift. The lower OAA reduces the aircraft's parasitic drag, which consequentially saves fuel. Control of the aircraft's CG along its yaw and pitch axes is also accomplished by the correct fuel transfer between the aircraft's various fuel tanks. It is however, the aircraft's pitch axis CG that has the largest effect on the aircraft's OAA.
Many factors are accounted for in calculating of the desired longitudinal CG during cruise flight, such as number and distribution of passengers and crew, and weight and onboard location of items such as freight, food, potable water, etc. All of these items together with the aircraft's unfueled, or “dry”, weight define the aircraft's zero fuel weight (ZFW). Adding the fuel on board (FOB) necessary for the flight brings the aircraft to its maximum take-off weight (MTOW). While the aircraft's ZFW weight remains relatively constant throughout the flight, the MTOW continuously decreases as fuel is consumed by the engines, thus making the aircraft's longitudinal CG dynamic. Fuel usage typically results in the longitudinal CG moving further aft to an undesired location.
Referring to FIG. 1 typical long range aircraft 200 carry their FOB in center main tanks 202, typically the largest tanks of the aircraft 200. Fuel is also carried in wing tanks, such as wing tanks 204, 206, 208, and flows out via collector cells 214 located in the fuel tanks which are directly supplying fuel to the engines. In this embodiment, there are four (4) engines in total (i.e. two (2) engines per wing as shown in FIG. 2). In some aircraft, additional fuel may be carried in tail plane tanks 210 and/or vertical tail fin tanks 212. Fuel is typically first consumed from the center main tanks 202, then from the wing tanks 204, 206, 208, beginning with inner wing tanks 204. Fuel in the outer wing tanks 208 is typically the last to be consumed, as the weight of the fuel in those tanks counteracts the increased wing lift force acting on the wing tips, thus reducing structural stresses on the wing tips during cruise flight conditions. Fuel surge tanks 216 in the wing tips of the aircraft are used to allow for any fuel volume change/spill-over from the outer wing tanks 208. Such fuel volume change may result (but it is not limited to) changes in fuel temperature and pressure, changes in ambient air temperature and pressure, changes in aircraft flight attitude (i.e. banking during turns, pitching up/down during take-off/landings, vibrations due to turbulence, etc.), etc.
Fuel is often transferred between the center main tanks 202 and the wing tanks 204, 206, 208 to maintain lateral trim, or balance, of the aircraft 200 as fuel is consumed. Also, fuel is transferred back and forth between the center main tanks 202 and the tail plane tanks 210 to affect the CG of the aircraft 200. Transferring fuel forward to the center main tanks 202 moves the CG forward, while transferring fuel rearward to the tail plane tanks 210 moves the CG rearward. This fuel transfer is typically performed manually by a member of the flight deck crew, often one dedicated to performing the fuel transfers. As the CG moves during flight due to fuel consumption, the flight deck crewmember turns on pumps to transfer fuel, then turns the pumps off when the CG is moved to a desired location. This process is repeated periodically throughout the flight, resulting in imprecise, stepwise changes in the CG throughout the flight, relative to a mean aerodynamic chord (MAC) of the aircraft.